Photoionization Dynamics in Pure Helium Droplets Page: 22 of 46
This article is part of the collection entitled: Office of Scientific & Technical Information Technical Reports and was provided to UNT Digital Library by the UNT Libraries Government Documents Department.
Extracted Text
The following text was automatically extracted from the image on this page using optical character recognition software:
electrons, which bear the hallmarks of direct photoionization, the electron kinetic energy
distribution of the slow electrons is independent of photon energy. The shapes of the
distributions at 25 eV in Fig. 5 are very similar to those obtained previously at 23.8 eV
(where no direct ionization was seen), nor are any significant changes seen at photon
energies up to 27 eV. This decoupling of the kinetic energy distribution from the photon
energy implies that the slow electrons arise from an indirect ionization mechanism involving
significant electron-He interactions. In addition, the temperature dependence of the slow
electrons show that they are strongly correlated with droplet size, and like the fast electrons,
become prominent when the expansion conditions for the helium change from subcritical to
critical. Combined with the changes in the time-of-flight mass spectra, and hence the
fragment cluster-ion mass distribution, the appearance of the slow electrons is clearly
correlated with production of the largest helium droplets.
In our earlier work below IE(He),13 we proposed that the slow photoelectrons resulted from
a complex, indirect mechanism involving (a) excitation of a Rydberg state of He2* within the
droplet that autoionizes to form He2+ or a larger cationic core, and (b) "thermalization" of the
resulting electron via electron-He interactions prior to its leaving the droplet.. The
importance of Rydberg-like excitations in He droplets below IE(He) has been established in
the fluorescence excitation spectra by Mbller and co-workers,53'55 and the role of molecular
autoionizing states was first suggested by Frochtenicht et al.9 to explain the onset of
photoionization below IE(He). The idea of electron thermalization was motivated by the
observation that much of the eKE distribution below IE(He) could be fit using the functional
form for thermionic emission from a very cold cluster (T<6K).22
Upcoming Pages
Here’s what’s next.
Search Inside
This article can be searched. Note: Results may vary based on the legibility of text within the document.
Tools / Downloads
Get a copy of this page or view the extracted text.
Citing and Sharing
Basic information for referencing this web page. We also provide extended guidance on usage rights, references, copying or embedding.
Reference the current page of this Article.
Peterka, Darcy S.; Kim, Jeong Hyun; Wang, Chia C.; Poisson, Lionel & Neumark, Daniel M. Photoionization Dynamics in Pure Helium Droplets, article, February 4, 2007; Berkeley, California. (https://digital.library.unt.edu/ark:/67531/metadc900443/m1/22/: accessed April 25, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.